Despite what superking said, Jaivez was completely right. Dattebayo has NO actual meaning and is just tagged to the end of narutos sentences, its just like his dialect.
Naruto's style is '(verb stem)tte bayo!' For instance, wakattebayo is "I get it already" or nan dattebayo is "What (the heck) is that?"

K: That isn't really modeled on anything either. When I thought of childlike
speech, "dattebayo" instantly came to mind. It's like it's become a part of
Naruto, and even now it brings out his "mischievousness".
(End quote)

I guess it's just up to fans to determine for themselves what it truly means...

In the Japanese versions, Naruto often ends his sentences with "dattebayo" or "(verb)-ttebayo", and as such Naruto has a unique style of speech. In the Japanese language, an ending can be added with no meaning to make the tone sound more tough, childish, girlish, or many other changes depending on the many existing endings. "-ttebayo", which is not a well-known ending, has no literal meaning and cannot be translated, but carries the connotations of the speaker being uncultured, brusque, and seeming tougher than they really are.In the English anime and video games, he often says "Believe it!" to fit with the mouth flaps of "-ttebayo".

1. The fansubbers from the US that fansub NarutoNaruto Shippuuden and Bleach. Without them half of the world would die of boredom and anime conventions would all be cancelled.
2. A phrase said by uzumaki naruto in the Naruto and Naruto Shippuuden series. Has no meaning in any way and is also clarified by dattebayo that the phrase means nothing at all

dattebayo the word that naruto often ends his sentances with. verbaly, it is said as "-ttebayo"
dattebayo has no real meaning, and the closest english equivelent is a pirates "arrgh!"
in the english dub (which is constantly plauged by crazed otaku, despite it being an excelent dub and true to the original)it is translated as "belive it!" in order for the mouth flaps to fit.
The specific "dattebayo" is not well known to even much of the Japanese native population.
The "dattebayo" specific ending can be considered a colloquial verb ending that makes the speaker sound uncultured, brusque, and trying to seem tougher than they really are.

1. A non-profit fansubbing group with an almost frightening sense of humor and a rather amusing tendency to release several troll episodes from time to time. Currently in charge of mainly Naruto and Bleach. Very well-known as compared to most fansubbing circles.

2. A word Uzumaki Naruto from the anime series, "Naruto", has a habit of adding into his sentence. It has no meaning whatsoever. The mangaka, Masashi Kishimoto, states that he carved it into Naruto's personality in hopes of making him seem more childish and playful. Naruto gradually reduces the frequency of using this word after the timeskip.